Acute cardiac allograft cellular rejection remains a significant source of morbidity and mortality within the first year after heart transplantation. In the first year after transplantation, nearly 63% of patients experience at least one episode of cardiac rejection and approximately one third of these patients will have multiple episodes. The clinical symptoms of acute cardiac rejection are relatively nonspecific (fatigue, dyspnea, low grade fever). No noninvasive method exists for the diagnosis of acute cardiac rejection. Several methodologies have been studied including electrocardiography, echocardiography, nuclear imaging, and phosphorus spectroscopy without success. The current gold standard for the diagnosis of acute cellular rejection remains right ventricular endomyocardial biopsy. We propose applying functional genomics to the study of acute cardiac allograft cellular rejection. We hypothesize that large scale expression profiling of circulating peripheral blood mononuclear cells (predominantly T lymphocytes) will identify genes that can serve as reliable biomarkers of acute cardiac cellular rejection. In the initial bench phase of the project peripheral blood mononuclear cells would be harvested from heart transplant recipients during periods of immunolgical tolerance of the allograft (no rejection) and immunologic intolerance of the allograft (rejection) to determine whether unique gene expression patterns are associated with each state. In the latter phase of the project we hope to translate these profiles into an acceptable bedside test for acute cardiac allograft cellular rejection. In addition to developing a biomarker approach to the diagnosis of rejection in cardiac transplant patients, expression profiling has the potential to identify immunoregulatory pathways that can serve as new targets for immunosuppressive therapy (rational drug development).